Fix incorrect 'curl -d POST' calls

[barney2k7]

'curl -d POST' actually posts to the url with a four byte request body 'POST', while those urls should actually be posted to without a request body. 'curl -X POST' does this. Also adding '-H "Content-Length: 0" as required by the server

[prmerger-automator[bot]]

@barney2k7 : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[barney2k7]

The same issue also exists in the linked script, see the corresponding PR microsoft/appcenter-Xamarin.UITest-Demo#61